During recent years, a number of biological and clinical results have shown that treatment of cancer may be improved when heat is applied to the cancerous tissue, especially when the heat treatment is combined with traditional cancer therapies such as radiation. Statistics on more than 800 cases show that the frequency of complete response rises from 25% when radiation alone is used to 64% when radiation is combined with a hyperthermia treatment.
Clinical results, however, are limited by the ability of hyperthermia systems to selectively heat tumors without damaging surrounding healthy tissues. Thus, non-invasive surface applicators used in conventional hyperthermia systems often obtain only superficial heating (3-5 cm in depth from the surface) because the electromagnetic wave penetration is limited by surrounding muscle tissue due to attenuation in the microwave range or due to damage from heating by strong near-field effects at low radio frequencies.
Existing applicators used for hyperthermia, i.e., heating of cancerous tumors, primarily heat superficial tumors and seldom provide for a controllable heating profile. It would be desirable to provide for the heat treatment of tumors at depths within the patient, and also provide for the treatment of tumors at non-symmetrical locations. Since the applicator is applied adjacent a patient, a suitable applicator should be compact and should have an open structure giving easy access to the patient. Many existing applicators are unsuitable for such topical application.
Where large body volume are to be heated, one known applicator system comprises a coil about the body which is excited with a radio frequency current. Such a system has a fixed power density distribution in a body which closely approximates a parabola having zero power at the center of the body. It is apparent that the surface excitation must be substantial to provide adequate heating excitation at a deep location.
Yet another large volume or whole body applicator is described in U.S. Pat. No. 4,462,412 where substantially uniform heating is produced within a cylindrical biological tissue specimen. A plurality of radiating apertures are matched with the size of the body specimen. The body specimen is actually placed within the annulus of the applicator for substantially circumferential radiation. This radiationg system provides an annular phased array, where a number of applicators are provided in a fixed annular arrangement and are excited from a common source in a coherent fashion. The applciator is tuned to provide a broadband match to body absorption to minimize reflections from the body tissue back to the excitation source. This whole body application distributes power in the body tissue for deep heating, but the distribution is generally uniform and is not localized at the tumor region. Substantial heating currents may occur near body curvatures and adjacent the feet and head regions. An absorbing material may be provided to absorb excess radiation but with concomitant inconvenience to the patient and to the clinician operating the equipment.
A theoretical analysis, Morita and Andersen, "Near-Field Absorption in a Circular Cylinder from Electromagnetic Line Sources," 3 Bioelectromagnetics, 253-274 (1982), has suggested that circumferential line sources might enable a power density maximum to be internally localized. The study further suggested that the local relative power maximum might be moved within the cylinder of excitation by controlling the included angle of the distributed circumferential sources. However, only circumferential sources and the relative included angle of adjacent sources were considered for analysis.
A need has thus arisen for apparatus which can deliver electromagnetic radiation at deep locations in body tissue and at specific places where the application of heat is desired, with minimum heating at other locations; where the power density may be shifted around by electronic means; and which have a structure which provides for convenient application to the patient and use by the clinical staff.
The disadvantages of the prior art are overcome by the present invention, however, and an improved apparatus and focusing method are provided for localizing heating effects for hyperthermia treatments of tissue at deep body locations.